Polk County presents five‑year Walnut Creek watershed plan; Urbandale assigned modest capture and floodplain targets

Polk County staff on Tuesday presented a five‑year implementation plan for the Walnut Creek Watershed Management Authority and asked Urbandale to adopt local targets and coordinate on projects to improve water quality and manage stormwater.

"We have been working for the last year or so on a 5 year implementation plan for the Walnut Creek Watershed Management Authority," said Kathy Drewel, water resources outreach coordinator with Polk County, during a Lunch and Learn for the Urbandale City Council. The plan updates a 2015 watershed plan and sets specific, measurable goals for the next five years.

The plan lays out five broad goals: hold water on the landscape, preserve and reconnect floodplains, improve surface water quality, strengthen regional collaboration and sustain implementation through monitoring and reporting. Polk County staff said the authority expects to support local projects, help secure grant funding and provide technical assistance to member cities.

For the watershed as a whole, the plan proposes a capture target equivalent to 50 acre‑feet per year (250 acre‑feet across five years). Urbandale’s share is set at 5 acre‑feet per year, reflecting that roughly 38% of the city’s area lies inside the Walnut Creek watershed, Drewel said. The plan also lists a watershedwide goal of 350 acres of floodplain improvement (70 acres per year) and a local Urbandale target of 16 acres, with an urban area share cited as 23%.

Polk County staff said they will work with Drake University on an E. coli monitoring plan and that the Watershed Management Authority (WMA) will hold quarterly check‑ins to track progress. "We're going to review the final plan with our Watershed Management Authority members on July 21," Drewel said, and staff expect implementation to begin in 2026 and run through 2031.

Staff outlined technical and outreach support the WMA can provide. The county is expanding its team: Polk County expects to hire a watershed planner and a construction/survey specialist, and the Soil and Water Conservation District is recruiting an additional urban conservationist to support residential outreach and project implementation.

Polk County staff described project examples intended to meet the plan goals. Soil quality restoration (SQR) uses a deep‑tine aerator to remove plugs and fill them with organic compost to improve infiltration. The county promotes a "batch and build" model in which municipalities contract a single contractor to treat many yards at once, lowering per‑yard costs and simplifying rebates. Drewel and Cassie Jewels of Polk County Soil and Water Conservation said batch programs have produced large takeups in nearby communities: the City of Johnston had 209 participating properties in one year; Des Moines had 227 properties treating nearly 800,000 square feet of urban land.

"Soil quality restoration is one of the easiest residential stormwater practices that can be done," Drewel said, explaining that deeper grass roots improve infiltration and reduce runoff entering storm systems.

Other project examples included oxbow and stream restorations, stormwater wetlands converted from older detention basins and native landscaping or prairie swales that reduce runoff and provide habitat and public amenities. Drewel pointed to an oxbow restoration at Waterford Park and a wetland retrofit in Grimes as models that combine flood management with public access and interpretation.

Council members asked staff to clarify technical definitions and local support. Council Member Kroll asked whether stormwater wetlands and detention basins count toward the capture goal; staff said McClure Consultants and the WMA are still refining definitions and will present further details to members. Jennifer Welch, urban conservationist with the Soil and Water Conservation District, described the six MS4 minimum control measures that communities meet through education, outreach, illicit discharge detection, erosion and sediment control, permitting and related activities.

Polk County staff also described grant structures and reimbursement rules. Under current local stormwater grant guidance, SQR projects receive a 50% match capped at $2,000 for a single‑family home; commercial projects may be eligible for different funding. Batch and build programs reduce resident paperwork and have produced lower per‑square‑foot costs for residents—Drewel said single projects can run about 22–25 cents per square foot, while batch bids have gone as low as 11 cents per square foot.

The WMA staff emphasized that the five‑year plan is intended to be "defined but not prescriptive": member cities are expected to set local implementation strategies that fit their land ownership and planning constraints, and WMA staff will provide assistance identifying sites, securing grants and coordinating projects.

Next steps: the WMA will finalize the plan at its July 21 meeting, continue quarterly monitoring and begin year‑one implementation in 2026. Polk County staff said they will share refined definitions for capture accounting and floodplain improvements and are available to meet with Urbandale staff to identify local project sites.

Sources: Remarks by Kathy Drewel (water resources outreach coordinator, Polk County), Cassie Jewels (Polk County Soil and Water Conservation), and Jennifer Welch (Soil and Water Conservation District) during the Urbandale Lunch and Learn. No formal council action on the plan was taken at this meeting.